In the part where Ace is making the gorilla sounds to lure away the poachers, the tall one says "Sounds like a female silverback" or something. A silverback is a name given to any dominant MALE and only male gorilla.There is no such thing as a silverback female.
Silverback is a race of gorilla, the reason the species is called this is because of the male's silverback. So technically you can say a silverback female, referring to the gorilla species' call.
It is true that only a male can be considered a silverback becuase of status in the gorilla troupes, but this could quite possibly be intentional to show that Ace isn't quite as educated about animals as he claims.
I quote from spud :"lure away the poachers, the tall one says "Sounds like a female silverback" " Spud said it was the poacher who said female siverback....not Ace. So quit with the blaming Ace. I think he proves himself about his knowledge of animals in the first Ace movie when he explains the difference between a porpoise and a dolphin.
Another user already mentioned that the silverback gorrilla is a term describing a male gorilla, i am not sure about simian hierachy but i believe that the silverbakc is the dominant male. (is this right?)
The fact of the matter is, that as dominant male there could be no such thing as a female dominant male gorilla, therefore it would be impossible for ace to emulate the call of a female silverback, given that the poachers are pretty thick, i'd say that the fault is in thier corner, not aces.